The present invention relates to a switching system for connecting a select antenna to a receiver, and more particularly to a hybrid antenna system for use with a VLF/Omega navigation system in which the better performing antenna of a dual antenna system is connected to a receiver.
A VLF/Omega navigational system is a world-wide system for assisting the navigation of airplanes and ships. The system includes a plurality of base stations located at selected points throughout the world. Each base station transmits an information signal having a unique frequency which is associated with the location of the station. The VLF/Omega information signals are in the low frequency range of 10-24 KHz. An aircraft using the navigational system receives one or more of the signals transmitted by the base stations. A computer on board the aircraft determines the phase or time of arrival of signals from three or more stations and computes the vehicle's location by trilateration, or hyperbolic computations.
The information signals which are transmitted by the base stations can be received by either a magnetic field (H-field) antenna or an electrical field (E-field) antenna. It is generally desirable to utilize an E-field antenna to receive the information signals, since this type of antenna is more sensitive and provides a stronger signal to the receiver. However, the E-field antenna is also sensitive to impulse noise, such as precipitation static, which has a relatively strong electrical field but a weak magnetic field. Precipitation static is caused when an aircraft flying through clouds containing particles such as ice crystals which become highly charged as a result of triboelectric or frictional charging, resulting in a corona discharge from some portion of the aircraft whenever the DC potential of the charge exceeds a threshold value. Each corona discharge results in static which is received by the receiver. The resulting precipitation static can take place at rates in the audio frequency range, and therefore interfere with the operation of the navigational system.
Impulse noise occurs frequently enough to create significant problems when using the navigational system. Consequently, most aircraft utilizing the VLF/Omega navigation system employ an H-field antenna to receive the information signals, since this type of antenna is nearly immune to the effects of impulse noise such as precipitation static. However, the use of such an antenna is not as sensitive to the low frequency information signals and therefore provides a weaker signal. In addition, the H-field antenna is susceptible to on-board electromagnetic interference, which causes further weakening of the information signals.
It is therefore an object of the present invention to provide a novel antenna system which makes optimum use of the desirable characteristics of E-field and H-field antennas.
A radio receiving apparatus which is responsive to both electrical and magnetic field components of a transmitted signal is disclosed in U.S. Pat. No. 3,475,687, issued to J. R. Pierce. In the apparatus disclosed in this patent, the signals from an E-field antenna and two orthogonal H-field antennas are combined and passed to a receiver. The disclosed apparatus does not include a provision for switching between the various antennas, and therefore has the drawback that the disadvantages as well as the advantages of the two types of antennas will have an effect upon the resulting output signal. Thus, the output signal will have static as a result of the sensitivity of E-field to impulse noise and will be weakened due to the sensitivity of the H-field antenna to electromagnetic interference, among other things.
It is therefore another object of the present invention to provide a novel hybrid antenna system which makes optimum use of two diverse types of antennas and suppresses the undesirable effects of the antennas and upon the resulting output signal.
An antenna system which includes a device for selecting between the output of one of a plurality of H-field antennas or an E-field antenna is disclosed in U.S. Pat. No. 3,718,932, issued to K. Ikrath et al. In the disclosed device of this patent, the selection device consists of manually-operated switches. The manually-operated switches are not desirable for use in the environment of a navigation system, however. For example, the output signal from the E-field antenna may be initially selected. When impulse noise occurs, an operator may disconnect the E-field antenna and connect one of the H-field antennas to a receiver system. In this situation, however, the operator will not know when the impulse noise has ceased, and therefore will not be able to reconnect the E-field antenna at the appropriate time. Instead, he must periodically disconnect the H-field antenna and connect the E-field antenna if he wishes to determine whether the impulse noise continues to be present.
It is therefore a further object of the present invention to provide a novel hybrid antenna system in which the selection of the better performing one of two antennas is automatic.
It is yet another object of the present invention to provide a novel hybrid antenna system in which at least one component of a received electromagnetic signal is continuously monitored to control connection of a select antenna to a receiver in an optimum manner.
Systems for selecting between one of two or more received signals and applying one of the signals to a receiver in response to certain signal quality criteria are disclosed in U.S. Pat. Nos. 1,934,211 and 2,968,718, for example. In the systems of these patents, the quality of signal strength or signal-to-noise ratio is evaluated and used to determine which of a plural number of received signals are to be passed on to a receiver. These systems do not relate to the detection of impulse noise and are not suitable in an environment such as a navigational system, for example, in which it is desirable to utilize the signal from a predetermined antenna under normal conditions and to apply the signal from a second antenna to a receiver only under certain circumstances, such as the occurrence of impulse noise.
It is therefore another object of the present invention to provide a novel hybrid antenna system which is particularly adapted for use in navigational systems.
It is a further object of the present invention to provide a novel hybrid antenna system in which a predetermined antenna is normally connected to a receiver except under certain circumstances.
These as well as other objects and advantages of the present invention will be apparent to one of ordinary skill in the art from a perusal of the following description when taken in cojunction with the accompanying drawings.